The present invention relates to a multi-layer, cros-extruded, thermoformable sheet suitable for the production of packaging material.
In particular the present invention relates to a co-extruded, rigid or semi-rigid, sheet comprising at least two polypropylene foam layers, said sheet being thermoformable into shaped articles for packaging.
Thermoforming is a common method for forming trays to be used in the packaging art for holding the products to be packaged. In some cases pre-formed trays are fed to a filling machine which inserts or lays the product to be packaged onto the tray and then either seals on a lid or wrap the loaded tray up. In most of the cases, particularly in the packaging of food products, e.g. fresh red meat or processed meat, a horizontal thermoform-fill-seal machine is employed. Said machine typically requires two separate webs of material, a lower or bottom web and an upper or top web. The lower web must be designed to be easily thermoformable as the machine creates a more-or-less shallow tray therefrom by pulling it across a heated cavity, or die, into which the sheet is drawn by vacuum at temperatures high enough to soften the sheet and allow it to adopt the shape of the cavity without tearing or cracking.
The sheet employed for this purpose therefore must have a good balance between thermoformability, as the speed of the thermoform-fill-seal machine is limited by the time required in the thermoforming step and the appearance of the end package will mainly depend on that of the supporting tray, and stiffness, as the tray should not bend during the packaging process or be distorted by the weight of the packaged product.
In some instances, when a lid is sealed to the tray or the tray rim, the lower web must also be heat-sealable. In some cases it is also required that said thermoformable sheet has a low permeability to gases, in particular oxygen, in order to prolong the shelf-life of oxygen susceptible products, e.g. most food products.
Other packaging machines, where an in-line thermoformable lower web is required, are those employed for skin packaging. In these machines the lower web is thermoformed into a tray, the product to be package loaded thereon and the upper web is drawn down by vacuum all around the product until the film conforms so faithfully to the product contours that it becomes like a skin. Also in this case a thermoformable sheet with a good balance between thermoformability and stiffness is required that preferably has gas barrier properties.
Cast solid sheets are typically employed for these applications. In some cases also foamed sheets are used, as the appearance of a foamed material is well appreciated in the market. One of the problems met with the foamed sheets is to obtain the desired balance between thermoformability and stiffness.
It is an object of the present invention to provide a rigid or semi-rigid foam sheet material that can suitably be thermoformed into a shaped container, such as a tray.
Another object of the present invention is to provide a thermoformable, multi-layer, co-extruded, polypropylene foam sheet having a density ranging from about 0.50 to about 0.85 g/cm3 and a flexural modulus higher than 400 MPa.
A still another object of the present invention is to provide a thermoformable foam sheet material co-extruded with a gas barrier layer for all those applications where a gas barrier packaging material is desired.
It has been found that the above objectives can be achieved by a multi-layer, co-extruded, sheet of thermoplastic material comprising at least two separate layers of foamed polypropylene obtained by chemically foaming polypropylene resins with different flexural modulus.
Polypropylene foam sheets comprising two different layers of foam polypropylene are described in WO 91/13933. More particularly said patent application describes polypropylene foam sheets obtained by physical foaming of a polypropylene resin with certain specific characteristics in terms of M2, M2/MW ratio, Je0 (equilibrium compliance) and/or S1/S (recoverable shear strain per unit stress). The process there described is a physical process, i.e. a process where foaming is obtained by the use of a blowing agent such as a hydrocarbon, e.g. butane and isopentane, a chlorinated or chloro-fluorinated hydrocarbon, e.g. chloro-difluoromethane, or an inert gas such as nitrogen or carbon dioxide. Generally, however, the use of a hydrocarbon either or a blend of a hydrocarbon with an inert gas obtains physical foaming. This creates problems of safety during the manufacturing process as well as during stocking of the foamed material. It is in fact necessary to age the foamed material in the warehouses until the trapped blowing gas is evolved and has been replaced by air. Forced ventilation of the warehouses is required and, depending also on the type of blowing agent employed, 5 to 7 days of aging are necessary before the foamed product can be distributed to customers.
WO 91/13933 also refers to the manufacture of a multi-layer foam sheet comprising a functional layer sandwiched between two polypropylene foam layers, possibly with tie layers bonding the foam polypropylene layer to the functional one. The process there schematically described provides for a co-extrusion where the same polypropylene material is foam extruded into two different layers and the foaming is always a physical foaming, with the drawbacks indicated above.